Detectors, as the name implies, are used to detect a characteristic of a measurable phenomenon. In electrical circuits, particularly communication circuits, it is often desirable to measure the power, current, or voltage of a signal at an intermediate point between the signal source and destination.
In microwave applications, a directional coupler or signal divider is used to generate a secondary signal that is the same or proportionally the same as the primary signal. The power level, for instance, of this secondary signal is then measured to determine the power level of the primary signal.
Conventionally, diode detectors are used in a monolithic circuit. In its simplest form, this could be a rectifier, such as a diode, coupled to ground through a capacitor. The output signal is then taken between the diode and capacitor. These do produce a signal representative of the primary signal, but they function best at low voltages.
It is therefore desirable to have a detector that is functional over a broader range of voltages, as well as being constructable as a monolithic integrated circuit. Considering that microwave systems find use in many diverse environments, it is also desirable to have such a detector that compensates for any variations in circuit operation due to variations in the temperature of the components.
The present invention variously provides these features. In particular, the present invention provides a monolithic detector using an active device for generating an output signal representative of an A.C. input signal. This is accomplished by using the functional characteristics of active devices without relying on the use of rectifiers.
In its general form, the present invention provides a first transistor having a control terminal for receiving the input signal, and two current-conducting terminals. One current-conducting terminal is coupled to a first reference voltage. A first impedance couples the other current-conducting terminal to a second reference voltage. The first and second reference voltages are appropriate for biasing the first transistor in a manner generating a first output signal on the other current-conducting terminal. A first capacitor couples the other current-conducting terminal to the first reference voltage for removing alternating currents from the first output signal.
For lower signal level applications, a generator generates a control voltage on the control terminal of the first transistor. A second transistor has a control terminal coupled to the generator for receiving the control voltage. One of two current-conducting terminals of the second transistor is coupled to the first reference voltage. A second impedance couples the other current-conducting terminal of the second transistor to the second reference voltage. Thus, a second output signal is generated between the second impedance and the second transistor such that the difference between the first and second output signals represents the power of the input signal.
In the preferred embodiment of the invention, an input capacitor is coupled to receive the input signal for transmitting only an alternating current input signal. A first common-source FET pair have gate terminals coupled to a temperature-compensating circuit for varying the gate-to-source voltage to compensate for operation variations in the transistors due to temperature changes. The gate of one FET is coupled to the input capacitor. The drain of the one FET is coupled through an intermediate capacitor to ground. That drain is also coupled to a relatively positive voltage source through a series-connected inductor and resistor. The drain of the second FET is also coupled to the voltage source through a resistor. An output capacitor couples the alternating current signal at the junction between the resistor associated with the first transistor and the inductor to ground. A differential amplifier generates an output signal representative of the difference between the signals appearing on the transistor sides of the two resistors for driving a comparator. The comparator output signal is representative of the input signal power.
These and other features of the invention are apparent from the preferred embodiment disclosed in the following detailed description and the associated drawings.